1. Field of the Invention
The present invention relates to a light emitting diode (LED). In particular, the present invention discloses an LED having a dual dopant contact layer.
2. Description of the Prior Art
The light emitting diode (LED) has been widely used in various fields. For example, the light emitting diodes are capable of being installed on optical display devices, traffic lights, data storage devices, communication devices, illuminative equipment, and medical equipment. Currently, an important issue for those skilled in the art is to improve electric characteristics and brightness of the LED. An LED structure with a thin nickel-gold (Ni/Au) transparent metallic layer forming on a p-type contact layer has been disclosed in the U.S. Pat. No. 5,563,422 for increasing light emitted from the LED. However, the transparent metallic layer made of the above-mentioned metallic materials merely has transmittance in the range of 60%˜70%. It not only affects light emitting efficiency of the LED, but also cannot provide a good current spreading effect because of its thickness being usually about 10 nm.
In order to solve the above-mentioned problem, an LED with a transparent conductive oxide layer formed on a p-type contact layer having high carrier concentration has been disclosed in U.S. Pat. No. 6,078,064. Because the transparent conductive oxide layer has high transmittance, the transparent conductive oxide layer can be thicker to better spread current in the transparent conductive oxide layer. Therefore, brightness of the LED can be increased by improving light-emitting characteristic of the LED. However, the carrier concentration of the p-type contact layer needs to be greater than 5*1018 cm−3 to form a good ohmic contact upon the transparent conductive oxide layer. With regard to the conventional semiconductor process, the p-type contact layer having high carrier concentration, however, is not easy to manufacture. It is well-known that the p-doped layer generally contains more defects, and the hydrogen atoms will affect formation of the p-doped layer. The carriers with high concentration are not easy to obtain even if a large amount of p-type dopants are implanted. Though this art can effectively increase light intensity of the light emitted from the LED, contact resistance between the p-type contact layer and the transparent conductive oxide layer is so high that the forward bias voltage of the LED can adversely affect electric characteristics of the LED.
An n+ reverse tunneling contact layer has been disclosed in Taiwan Pat. No. 144415 assigned to the same assignee as the present invention. The n+ reverse tunneling contact layer is positioned between a transparent electrode layer and a semiconductor light emitting layer, and a tunneling mechanism is utilized to form an ohmic contact associated with the transparent electrode layer and the semiconductor light emitting layer. The n+ reverse tunneling contact layer, unlike the above-mentioned p-type contact layer, having high carrier concentration is utilized to reduce difficulty in manufacturing the LED. However, the n+ reverse tunneling contact layer is sensitive to its thickness and concentration of the n-type carriers. When the concentration of the n-type carriers is too low, or the n+ reverse tunneling contact layer is too thick, the tunneling mechanism is blocked. Therefore, the formation of the n+ reverse tunneling contact layer needs to be strictly controlled.